1. Field of the Invention
The present invention relates to a channel port module (CPM), and more particularly, to a channel port module or terrestrial interface module (TIM), capable of being bridged in parallel with other channel port modules employed at a remote satellite earth station handling synchronous sub-T1 traffic from a plurality of separate users in a manner whereby separate multiplexers-demultiplexers are not required at the hub office earth station to communicate with the plurality of separate users.
2. Description of the Prior Art
Conventionally, the equipment at an earth station is divided into equipment which is commonly employed for the entire earth station and equipment which is unique to each of the various information ports. A typical example of this can be found in U.S. Pat. No. 3,838,221 issued to Schmidt et al on Sept. 24, 1974, wherein the common equipment includes transmit and control equipment, a multiplexer and a demultiplexer, and a plurality of terrestrial interface modules (or TIMs) which are unique to each of the different information ports. A consequence of this arrangement of hardware at a TDMA earth station is a large amount of equipment duplication. More particularly, the equipment employed to change the information bit rate employs a pair of memories. In a first TDMA frame, information is written continuously into one of the memories at a bit rate the same, or nearly the same, as the rate at which the information is received. Simultaneous with writing into one of these memories, the other memory is prepared for, or actually engaged in, reading out the information previously written therein at a much higher rate. On the next frame the function of the memories is interchanged so that, while the first memory is read, the second is being written into. These functions require an address counter for properly storing and retrieving the received information and pulse sources to operate these addressing counters. Thus, each of the TIMs employed must contain equipment to perform these functions.
An alternative arrangement wherein the TIMs are essentially eliminated as stand-alone devices and instead their functions are incorporated within the multiplexer-demultiplexer is disclosed in U.S. Pat. No. 4,224,473 issued to Kaul et al on Sept. 23, 1980. Incorporation of this equipment simultaneously allows elimination of the many high speed line buffers and drivers previously required to connect various control signals from the multiplexer-demultiplexer to the plurality of TIMs, and also eliminates much of the equipment duplication by employing common equipment in the multiplexer-demultiplexer to perform the functions at the various ones of the ports in the multiplexer-demultiplexer. The multiplexer-demultiplexer is capable of converting plural asynchronous terrestrial signal inputs into a burst signal or signals for transmission and for converting a received burst signal into plural asynchronous terrestrial signals for coupling to a terrestrial network and includes a plurality of input ports, each of the input ports including an elastic buffer for writing therein in response to data received at the port at a rate commensurate with the rate of receipt of data at the port.
Many earth station arrangements comprise a plurality of separate earth stations which communicate with the users via existing T1 (1.5 Mb/s) lines. However, most communications to be transmitted over these T1 lines originate with users which operate at a sub-T1 rate, usually 64 kb/s (termed DSO rate), and are, for the most part, synchronous signals. Therefore, the above-described Kaul et al arrangement requires additional multiplexing equipment besides that which is required to establish earth station-to-earth station communication in a TDMA environment.
The problem remaining in the prior art, therefore, is to provide a means for efficiently utilizing the existing T1 facilities without requiring a separate multiplexer-demultiplexer at each interface unit.